1. Field of the Invention
The present invention relates generally to a photoprocessing method for processing an object to be processed using a laser beam, and more particularly, to a photoprocessing method used for processing photoelectric converting elements in a photovoltaic device to fabricate the photovoltaic device or crystallizing an amorphous semiconductor film.
2. Description of the Prior Art
A photoprocessing method using a laser beam has been conventionally employed in order to finely process photoelectric converting elements in a photovoltaic device to fabricate the photovoltaic device or crystallize an amorphous semiconductor film.
As such a photoprocessing method using a laser beam, a YAG laser processing method using YAG laser having a fundamental wavelength of 1.06 .mu.m has been conventionally utilized widely.
In the conventional YAG laser processing method, a laser beam in a spot shape is irradiated onto an object to be processed. For example, when a photovoltaic device constructed by electrically connecting a plurality of photovoltaic converting elements 20 in series is fabricated on the surface of an insulating substrate 11, as shown in FIG. 1, a laser beam in a spot shape irradiated from a laser device 1 is generally reflected from a reflector 2, the laser beam thus reflected is gathered by a lens 3 and is irradiated onto an object to be processed, the laser beam is scanned in the direction of processing to perform groove processing on the object to be processed, and such scanning is repeated, thereby to form a plurality of grooves in the object to be processed.
The steps of fabricating the above described photovoltaic device will be specifically described.
A continuous first electrode film 12 is first formed on the surface of the insulating substrate 11, as shown in FIG. 1(A). The laser beam in a spot shape irradiated from the laser device 1 as described above is reflected from the reflector 2, the reflected laser beam is gathered by the lens 3 and is irradiated onto the first electrode film 12, and the laser beam is scanned, to perform groove processing for forming a plurality of grooves on the first electrode film 12, so that the first electrode film 12 is divided for each photoelectric converting element 20, as shown in FIG. 1 (B). A continuous photoelectric converting layer 13 is then formed on the substrate 11 in which the first electrode film 12 is divided, as shown in FIG. 1(C). Thereafter, groove processing for forming a plurality of grooves is similarly performed on the photoelectric converting layer 13, so that the photoelectric converting layer 13 is divided for each photoelectric converting element 20, as shown in FIG. 1(D). A continuous second electrode film 14 is further formed on the divided photoelectric converting layer 13, as shown in FIG. 1(E). Thereafter, groove processing for forming a plurality of grooves is similarly performed on the second electrode film 14, so that the second electrode film 14 is divided for each photoelectric converting element 20, as shown in FIG. 1(F). The above described photovoltaic device is thus fabricated.
When the laser beam in a spot shape is successively scanned in the direction of processing as described above, to perform groove processing with spots of the laser beam successively continued, however, the scanning speed or the like must be suitably controlled depending on a material to be processed, whereby the control is difficult, and the laser beam must be scanned for each groove processing for forming one groove. If the photovoltaic device requiring groove processing for forming a lot of grooves is fabricated as described above, therefore, it takes a lot of time to fabricate the photovoltaic device, resulting in very poor productivity.
Furthermore, when the groove processing is performed with the spots of the laser beam in a spot shape successively continued as described above, a portion where the spots of the laser beam are overlapped with each other occurs, as shown in FIG. 2. The laser beam is irradiated twice in the portion where the spots are thus overlapped with each other. Therefore, the object to be processed is thermally affected in the portion where the spots are thus overlapped with each other of the laser beam, whereby some problems arise. For example, the object to be processed is degraded in the portion.
Additionally, in the case of YAG laser conventionally used, its optical energy is as low as 1.23 eV, whereby processing using the laser beam becomes difficult depending on the material of the object to be processed. In fabricating the above described photovoltaic device, groove processing cannot, in some cases, be performed on the first and second electrode films 12 and 14 and the photoelectric converting layer 13.
In recent years, therefore, a photoprocessing method of expanding a pulse laser beam by a beam expander to increase the area thereof, gathering the laser beam the area of which is thus increased by a cylindrical lens or the like to make the laser beam linear, and irradiating the linear laser beam onto an object to be processed to perform groove processing on the object to be processed, as disclosed in Japanese Patent Laid-Open No. 206558/1993, has been developed in order to solve the above described problems.
When the laser beam thus expanded by the beam expander is gathered by the cylindrical lens or the like, however, the Gaussian distribution occurs in the energy intensity of the laser beam by the light gathering. Consequently, energy in the center of the laser beam becomes higher than energy in the periphery thereof. When the laser beam thus gathered is irradiated onto the object to be processed to perform processing, abnormalities occur in the center and the periphery of a processed portion on which the laser beam is irradiated.
For example, in fabricating a photovoltaic device constructed by electrically connecting a plurality of photoelectric converting elements 20 in series on the insulating surface of the substrate 11 as described above, when the first electrode film 12 continuously formed on the substrate 11 such as a glass substrate or an organic film substrate is subjected to groove processing, and the first electrode film 12 is divided for each photoelectric converting element 20, the laser beam gathered to be made linear as described above is irradiated onto the first electrode film 12. When the first electrode film 12 is removed from above the substrate 11 in a portion on which the laser beam is irradiated to perform groove processing, the substrate 11 under the first electrode film 12 is thermally damaged to be degraded in a portion 11a on which the laser beam in the center having high energy is irradiated, whereby a microcrack occurs in this portion 11a.
Consider a case where the photoelectric converting layer 13 such as an amorphous semiconductor film is continuously formed on the substrate 11 in which the first electrode film 12 is divided as described above, after which groove processing is performed on the photoelectric converting layer 13, to divide the photoelectric converting layer 13 for each photoelectric converting element 20 and expose the first electrode film 12 connecting the adjacent photoelectric converting elements 20 in series. In this case, if the laser beam gathered to be made linear is irradiated onto the photoelectric converting layer 13 to perform groove processing as described above, the energy in the center of the laser beam and the energy in the periphery thereof differ from each other, whereby various problems as shown in FIGS. 4(a) to 4(C) arise.
Specifically, in the above described laser beam, the energy in the center thereof is higher than the energy in the periphery thereof. Even if the photoelectric converting layer 13 in the portion on which the laser beam in the center is irradiated is successfully removed, therefore, the photoelectric converting layer 13 is not successfully removed in the portion on which the laser beam in the periphery having low energy is irradiated, and the photoelectric converting layer 13 in this portion is annealed, to be finely crystallized or crystallized, whereby low resisting portions 15a and 15b are formed, as shown in FIG. 4(A). Even if the photoelectric converting elements 20 are separated from each other after a second electrode film 14 is formed on the photoelectric converting layer 13, therefore, the first electrode film 12 and the second electrode film 14 are coupled to each other and are short-circuited by the above described low resisting portion 15b within one of the photoelectric converting elements 20.
Furthermore, the photoelectric converting layer 13 is not sufficiently removed in the above described portion on which the laser beam is irradiated, so that a molten object 16 of the photoelectric converting layer 13 may, in some cases, remain in the portion on which the laser beam is irradiated, as shown in FIG. 4(B). Accordingly, it is impossible to accurately process the photoelectric converting layer 13 to have a predetermined pattern.
On the other hand, when the photoelectric converting layer 13 is removed also in the portion on which the laser beam in the periphery having low energy is irradiated, the first electrode film 12 under the photoelectric converting layer 13 is thermally damaged to be degraded in a portion 12a on which the laser beam in the center having high energy is irradiated, as shown in FIG. 4(C), whereby some problems arise. For example, the resistance of the first electrode film 12 is increased in this portion 12a.
Furthermore, even when the photoelectric converting layer 13 is divided between the adjacent photoelectric converting elements 20 as described above, after which the second electrode film 14 is continuously formed on the photoelectric converting layer 13 thus divided, the above described laser beam is irradiated onto the second electrode film 14 to perform groove processing, and the second electrode film 14 is divided between the adjacent photoelectric converting elements 20, the energy in the center of the laser beam and the energy in the periphery thereof differ from each other, whereby various problems as shown in FIGS. 5(A) to 5(D) arise.
For example, when the second electrode film 14 formed on the photoelectric converting layer 13 is subjected to groove processing by the above described laser beam, to divide the second electrode film 14 between the adjacent photoelectric converting elements 20, the energy in the center of the laser beam is higher than the energy in the periphery thereof. As shown in FIG. 5(A), therefore, the photoelectric converting layer 13 under the second electrode film 14 is annealed in the portion on which the laser beam in the center is irradiated, so that the photoelectric converting layer 13 in this portion is finely crystallized or crystallized to form a low resisting portion 15. As shown in FIG. 5(B), the second electrode film 14 is not removed and is melted in the portion on which the laser beam in the periphery having low energy is irradiated. The molten object 16 flows out. Portions into which the second electrode film 14 is divided are connected to each other by the molten object 16. Therefore, the second electrode film 14 cannot be reliably divided between the adjacent photoelectric converting elements 20.
Furthermore, in dividing the second electrode film 14 between the adjacent photoelectric converting elements 20, when the second electrode films 14 and the photoelectric converting layer 13 under the second electrode film 14 are subjected to groove processing by the above described laser beam, the second electrode film 14 and the photoelectric converting layer 13 are respectively divided because their respective parts are removed in the portion on which the laser beam in the center having high energy is irradiated, as shown in FIG. 5(C). However, the photoelectric converting layer 13 is annealed in the portion on which the laser beam in the periphery having low energy is irradiated, and the photoelectric converting layer 13 in this portion is finely crystallized or crystallized, to form low resisting portions 15a and 15b. As shown in FIG. 5(D), molten objects 16a and 16b of the second electrode film 14 melted by the laser beam flow out, to be connected to the first electrode film 12. Therefore, the first electrode film 12 and the second electrode film 14 are coupled to each other and are short-circuited within the same photoelectric converting element 20 by the low resisting portion 15b and the molten object 16b.
Additionally, as a technique for correcting defects occurring in forming a photomask on the object to be processed, which is not a method of directly processing the object to be processed by a laser beam, there exists a technique for restraining the magnitude of the laser beam through a rectangular slit formed in the mask, gathering the laser beam passed through the slit by a condensing lens and irradiating the laser beam onto a defective portion, thereby to correct the defective portion.
Also in this case, however, the laser beam gathered by the condensing lens is irradiated. When the laser beam thus gathered is directly irradiated onto the object to be processed to perform processing, therefore, abnormalities occur in the center and the periphery of a processed portion on which the laser beam is irradiated, as in the above described case.
Furthermore, the laser beam has been conventionally irradiated onto an amorphous semiconductor film, to crystallize the amorphous semiconductor film.
Also in this case, however, if the energy in the center of the laser beam and the energy in the periphery thereof differ from each other as described above, crystals are nonuniform when the amorphous semiconductor film is crystallized. Therefore, a good crystallized semiconductor layer having uniform properties is not obtained.